The present invention relates to improvements in machines and methods for making multi-ply webs from plural supplies of stock each having fibers suspended in a liquid. More particularly, the invention relates to an improved high speed forming arrangement which is particularly well adapted to making multi-ply paper web.
In the formation of multi-ply webs, machines to be competitive must cope with the problems of forming a stronger stiffer finished web having multiple plies and must operate at increasing rates of speed in a manner to either improve the characteristics of the finished web or at least not to detract from the characteristics. To achieve the characteristics of a good multi-ply web, an intermediate layer must be employed which lends increased strength and stiffness, and outer covering plies must be used which cover the intermediate ply and which present an improved smooth surface finish capable of having good appearance and having satisfactory and improved printing qualities.
Technology in paper making has led itself to the formation of multi-ply webs in a single operation wherein the plural plies are simultaneously delivered between opposed travelling forming wires rather than being formed in a layer at a time with the layers laid down on top of each other and bound to each other. A type of multi-ply web forming machine wherein the layers are simultaneously formed is illustrated in U.S. Pat. No. 3,923,593 to Verseput, issued Dec. 2, 1975. This patent illustrates the simultaneous forming of multiple plies with the center ply having a high ash content for improved qualities of strength.
In the instant invention, the problem of the high rate of water drainage which must be encountered is considered, and in accordance with the principles of the present invention, a center ply is formed of a high consistency stock reducing the water drainage which must occur from the intermediate layer through the outer layers. It has been discovered that by utilization of the method in accordance with the present invention, improved machine capacities and speeds are achieved and a higher quality multilayer board is obtained. By using a high consistency stock to form the middle ply, a finished board with greater bending stiffness has been achieved. Furthermore, a heavier higher basis weight board can be formed in a machine with no increase in the water removal capacity. Present commercial methods of forming three-ply board on a twin wire former results in a limitation in basis weight of on the order of 42 pounds per thousand square feet. Using a high consistency stock on the order of 2% to 5%, an increase in basis weight up to 90 pounds per thousand square feet has been obtained. It is also anticipated that bleach board could be made in the range of 33 to 110 pounds per thousand square feet.
One of the difficulties in the formation of multi-ply board with a higher consistency center layer has been the uneven formation and arrangement of fibers in the center board resulting in a bumpy surface on the outer layers and resulting in a less strong and less stiff finished board.
It is an object of the present invention to eliminate the problems of an uneven or bumpy surface board and to increase the overall stiffness of the board by treating the center or intermediate layer of stock in a different manner than the outer finished layers so that an energetic fine scale turbulence is maintained in the outer layers while the thicker inner layer is treated to a controlled turbulence by passing the intermediate layer of stock through a tortuous path of a unique configuration.
It is a further object of the invention to provide a new and improved method for the relatively high speed formation of multiply paper web, and particularly to provide for delivering multiple plies of stock between twin forming wires for continuous reliable improved formation.